Biosensors usually provide an analysis of a biological fluid, such as whole blood, urine, or saliva. Typically, a biosensor analyzes a sample of the biological fluid to determine the concentration of one or more analytes, such as glucose, uric acid, lactate, cholesterol, or bilirubin, in the biological fluid. The analysis is useful in the diagnosis and treatment of physiological abnormalities. For example, a diabetic individual may use a biosensor to determine the glucose level in blood for adjustments to diet and/or medication. When used, a biosensor may be underfilled if the sample size is not large enough. An underfilled biosensor may not provide an accurate analysis of the biological fluid.
Biosensors may be implemented using bench-top, portable, and like devices. The portable devices may be hand-held. Biosensors may be designed to analyze one or more analytes and may use different volumes of biological fluids. Some biosensors may analyze a single drop of whole blood, such as from 0.25-15 microliters (μL) in volume. Examples of portable measuring devices include the Ascensia Breeze® and Elite® meters of Bayer Corporation; the Precision® biosensors available from Abbott in Abbott Park, Ill.; Accucheck® biosensors available from Roche in Indianapolis, Ind.; and OneTouch Ultra® biosensors available from Lifescan in Milpitas, Calif. Examples of bench-top measuring devices include the BAS 100B Analyzer available from BAS Instruments in West Lafayette, Ind.; the CH Instruments' Electrochemical Workstation available from CH Instruments in Austin, Tex.; the Cypress Electrochemical Workstation available from Cypress Systems in Lawrence, Kans.; and the EG&G Electrochemical Instrument available from Princeton Research Instruments in Princeton, N.J.
Biosensors usually measure an electrical signal to determine the analyte concentration in a sample of the biological fluid. The analyte typically undergoes an oxidation/reduction or redox reaction when an excitation signal is applied to the sample. An enzyme or similar species may be added to the sample to enhance the redox reaction. The excitation signal usually is an electrical signal, such as a current or potential. The redox reaction generates an output signal in response to the excitation signal. The output signal usually is an electrical signal, such as a current or potential, which may be measured and correlated with the concentration of the analyte in the biological fluid.
Many biosensors have a measuring device and a sensor strip. A sample of the biological fluid is introduced into a sample chamber in the sensor strip. The sensor strip is placed in the measuring device for analysis. The measuring device usually has electrical contacts that connect with electrical conductors in the sensor strip. The electrical conductors typically connect to working, counter, and/or other electrodes that extend into a sample chamber. The measuring device applies the excitation signal through the electrical contacts to the electrical conductors in the sensor strip. The electrical conductors convey the excitation signal through the electrodes into a sample deposited in the sample chamber. The redox reaction of the analyte generates an output signal in response to the excitation signal. The measuring device determines the analyte concentration in response to the output signal.
The sensor strip may include reagents that react with the analyte in the sample of biological fluid. The reagents may include an ionizing agent for facilitating the redox of the analyte, as well as mediators or other substances that assist in transferring electrons between the analyte and the conductor. The ionizing agent may be an analyte specific enzyme, such as glucose oxidase or glucose dehydrogenase, which catalyze the oxidation of glucose in a whole blood sample. The reagents may include a binder that holds the enzyme and mediator together.
Biosensors may include an underfill detection system to prevent or screen out analyses associated with sample sizes that are of insufficient volume. Because concentration values obtained from an underfilled sensor strip may be inaccurate, the ability to prevent or screen out these inaccurate analyses may increase the accuracy of the concentration values obtained. Some underfill detection systems have one or more indicator electrodes that detect the partial and/or complete filling of a sample chamber within a sensor strip. The indicator electrode(s) may be separate or part of the working, counter, or other electrodes used to determine the concentration of analyte in the sample. An electrical signal usually passes through the indicator electrode(s) when a sample is present in the sample chamber. The electrical signal may be used to indicate whether a sample is present and whether the sample partially or completely fills the sample chamber.
Some biosensors have a third or indicator electrode in addition to the counter and working electrodes used to apply an excitation signal to a sample of the biological fluid. The third electrode may be positioned to detect whether the sample forms a liquid junction between the electrodes. In operation, a potential is applied between the third electrode and the counter electrode. When the sample connects the electrodes, current flows between the third and counter electrodes. The biosensor detects the current to determine whether the sensor strip is filled. A biosensor using an underfill detection system with a third electrode is described in U.S. Pat. No. 5,582,697.
Other biosensors use a sub-element of the counter electrode to determine whether the sensor strip is underfilled. The sub-element may be located upstream from the working electrode, where only the sub-element is in electrical communication with the working electrode when the sensor strip is underfilled. In operation, an insufficient flow of current between the sub-element and the working electrode occurs when the sensor strip is underfilled. The biosensor detects the insufficient flow of current and provides an error signal indicating the sensor strip is underfilled. A biosensor using an underfill detection system with a sub-element of the counter electrode is described in U.S. Pat. No. 6,531,040.
While these underfill detection systems balance various advantages and disadvantages, none are ideal. These systems usually include additional components, such as the indicator electrodes. The additional components may increase the manufacturing cost of the sensor strip. The additional components also may introduce additional inaccuracy and imprecision due to the variability of manufacturing processes.
In addition, these systems may require a larger sample chamber to accommodate the indicator electrodes. The larger sample chamber may increase the sample size needed for an accurate and precise analysis of the analyte.
Moreover, these systems may be affected by uneven or slow filling of the sample chamber. The uneven or slow filling may cause these systems to indicate that the sensor strip is underfilled when the sample size is large enough. The uneven or slow filling also may cause these systems to indicate the sensor strip is filled when the sample size is not large enough.
These systems also may not detect that the sensor strip is underfilled early enough to add more of the biological fluid. The delay may require replacing the sensor strip with a new sensor strip and a new sample of the biological fluid.
Accordingly, there is an ongoing need for improved biosensors, especially those that may provide increasingly accurate and/or precise detection of underfilled sensor strips. The systems, devices, and methods of the present invention overcome at least one of the disadvantages associated with conventional biosensors.